The invention is directed to a method and apparatus for heating and stretching a glass preform to produce a glass rod having a uniform, pre-selected diameter, while limiting excursions of the rod diameter from the pre-selected value. The novel method and apparatus begins control of the diameter of the glass rod as soon as a portion of the preform reaches a temperature which causes the preform portion to soften and begin moving downward under the influence of gravity.
An efficient method for making a draw preform, from which optical waveguide fiber can be drawn, includes first making a core preform. The core preform is heated and stretched into a plurality of core rods which serve as bait rods for depositing cladding glass. Waveguide fiber performance depends critically on the geometric uniformity and the dimensions of the core and cladding glass. Thus, careful control of preform geometry is required to insure proper geometry of the final waveguide fiber.
In particular, control of the geometry of the core rods drawn from the core preform is crucial. The roundness of the final waveguide fiber as well as the relative core to clad diameter ratio and concentricity depend upon control of the absolute core rod dimensions and the uniformity of the dimensions from point to point along the core rod. The step of drawing the core rod from the core preform has become even more important due to the recent increase in demand for very high performance waveguide fibers. Low attenuation and high bandwidth depend strongly on waveguide geometry. In addition, the cost of core dopants such as germanium continue to increase, thus providing motivation to improve utilization of the core preform, which contains essentially all of the dopant material used in the waveguide manufacturing process.
The most inefficient part of the core rod drawing process is in the initial step. The preform is lowered into a furnace until the preform tip and a portion of the preform adjacent the tip is within the furnace hot zone. The preform portion softens and a gob of glass begins to flow from the preform tip. As the glass gob stretches away from the preform tip, the gob of glass, which is suspended from the preform by a strand of glass, emerges from the furnace where it can be gripped by a mechanical device. Once the glass strand is gripped, the geometry control algorithm, which includes grip device velocity and tension in the glass, is activated and the diameter of the strand is brought to a target value. The target velocity refers to the velocity of a point on the glass rod measured at a time when the process is steady state and the diameter of the rod is nominal. The control parameters are:
furnace temperature; PA1 drawing (grip) velocity; PA1 drawing tension; and, PA1 feed rate of the preform into the hot zone.
The variables are closely interrelated. For example, a decrease in furnace temperature will cause drawing velocity to decrease unless the drawing tension is increased. If drawing tension is increased without increasing furnace temperature or downfeed rate, rod diameter will fall below target value. For proper diameter control, continuous measurements of diameter must be taken and the control parameters adjusted, usually semi-continuously. Typically, a computer is interfaced with the control devices of a preform drawing apparatus and an algorithm is employed to control the diameter of the drawn glass rod.
Control of the interactive drawing parameters is most accurate when the glass rod diameter is close to the target value, i.e., when only small changes in control parameters are needed. A problem intrinsic to the preform drawing process is the distance the initial gob of glass must travel from the preform before the connecting glass strand or rod can be gripped and fully controlled drawing can begin. The glass strand may have a diameter as much as a factor of ten smaller than the target diameter before the strand can be gripped and diameter control begin. The result is an appreciable fraction, as much as 25%, of core preform glass can be lost in the gob drop and the off target rod length drawn while the required large diameter correction is made.
Thus there is a need in the waveguide manufacturing process for an improved preform drawing method and associated apparatus which decreases preform glass losses, thereby decreasing cost and increasing equipment utilization, by controlling the diameter of the drawn rod from the beginning of glass gob drop, i.e., before the glass is gripped. In particular, greatest savings in time and materials is achieved when diameter control begins before the diameter of the glass strand stretching between the gob and the preform body has deviated far from the target diameter.